A butyrate kinase from Clostridium acetobutylicum has been purified 50-fold to homogeneity in a five-step procedure with a 31% yield. The purification involved ammonium sulfate fractionation, two hydrophobic interaction chromatography steps, affinity chromatography, and gel filtration. The isoelectric point, the molecular weights of the native and denatured enzyme, the pH optimum, the substrate specificity, and the amino acid composition of the enzyme have been determined. Antibodies to butyrate kinase are currently being prepared in a rabbit. These will be used to study the expression of the enzyme as a function of fermentation time.